Group of Seven leaders agreed Thursday on the need to send a strong message on maritime claims in the western Pacific, while, separately, U.S. President Barack Obama remarked world leaders are rightly "rattled" by the rise of Donald Trump.

Japanese Prime Minister Shinzo Abe "led discussion on the current situation in the South China Sea and East China Sea," Deputy Chief Cabinet Secretary Hiroshige Seko told reporters after a session on foreign policy affairs.

"Other G7 leaders said it is necessary for G7 to issue a clear signal."

The agreement prompted a sharp rejoinder from China, which is locked in territorial disputes with Japan and other neighbours and whose rise as a power has put it at the heart of some discussions at the advanced nations' summit in Ise Shima, in central Japan.

Chinese Foreign Ministry spokeswoman Hua Chunying retorted in Beijing that the South China Sea issue had "nothing to do" with the G7 or any of its members.

"China is resolutely opposed to individual countries hyping up the South China Sea for personal gain," she said.

On the summit's sidelines, Obama discussed the 2016 presidential campaign during a news conference, saying foreign leaders are surprised by the Republican front-runner and not sure how seriously to take the things he says.

The president said many of Trump's proposals display ignorance about world affairs, a cavalier attitude or an interest in getting "tweets and headlines."

U.S. President Barack Obama attends a news conference during the 2016 Ise-Shima G7 Summit in Shima, Japan. (Carlos Barria/Reuters)

Global economic risks

The G7 leaders sat down at a big round table for the first of their summit working sessions after strolling through the grounds of the Ise Shrine, a tranquil, densely forested landmark that is the holiest site in the Japan's indigenous Shinto religion, and then joining a group of children in a tree planting ceremony.

The G7 gathering dovetails in many ways with Abe's long-term diplomatic, political and economic agenda. A dramatic statement about global economic risks and a strong show of support for public spending to help spur growth could help Abe justify extra stimulus and possibly provide political cover for postponing an unpopular but badly needed increase in the sales tax next April.

An aide said Abe planned to use data charts dramatically illustrating the severity of the recent slump in commodity prices and the slowdown in China.

"There is a concern that the sluggish economy might last some time, and that Abe hopes to share a common notion about the potential risks," said Hiroshige Seko, a deputy chief cabinet secretary.

Abe hopes the group would "take leadership as the G7 and convey a powerful message to the rest of the world," he said. Seeking to enhance Abe's legacy, the message would be dubbed the "Ise-Shima" principle, he said.

The leaders were expected to turn their attention to trade, politics and diplomacy and on climate change and energy during talks later Thursday.

G7 leaders attend a meeting at the Shima Kanko Hotel in Shima, Japan, on Thursday. (Reuters)

Security and 'womenomics'

The annual summit brings together the leaders of Britain, Canada, France, Germany, Italy, Japan and the United States. It is taking place amid extraordinarily tight security around the remote summit venue, with uniformed police standing guard at close intervals on both sides of roads and randomly in forests, rice fields, soccer fields and other locations.

Many of the issues to be discussed during their two days of talks are linked to other Abe policy priorities. They include maritime security — code for concerns over China's expanding presence in disputed areas of the South China Sea; initiatives on global health, including funding for fighting terrorism and pandemics; and a focus on women's empowerment, which Abe has promoted as "womenomics."

Abe told his fellow G7 leaders on Thursday that North Korea's nuclear and missile programs are also a concern to Europe, according to a top Japanese government official.

The prime minister also said it is important to have Russia's constructive engagement in neighbouring Ukraine and said Japan is ready to extend a fresh $500 million in aid to Iraq, Deputy Chief Cabinet Secretary Hiroshige said.

Support for refugees

Japan planned to propose expanding support, including development assistance loans, for building roads, power plants and other infrastructure around the world, seeking a target of $200 billion in financing. Tokyo also plans to spend about $6 billion on education, training and job creation for 20,000 people in the Middle East, to help promote development and social stability in the region and counter the chronic unemployment and economic disparities that are contributing to extremism and violence, the Japanese government said.

On gender issues, Japan plans to help improve schooling conditions for 50,000 girls, mainly in Africa and South Asia, and provide training for 5,000 women in fields such as maternal and child health, it said.

Japanese officials have highlighted joint efforts on corruption, terrorism, global health and migration, which has become a huge headache especially for European nations, as other top priorities.

"Those who criticize us should rather think how to increase their assistance because what Europe provides is already massive," Donald Tusk, president of the European Council, calling for G7 support and leadership. "And honestly speaking, if they don't take the lead in managing this crisis, nobody else will. I will appeal to G7 leaders to take up this challenge."

Tusk said the EU was seeking more support for refugees and creation of resettlement schemes and other forms of legal migration around the world.

A possible exit from the European Union by Britain, depending on a June 23 vote, is also hanging over the talks.

Obama arrived in Japan on Wednesday and had an evening meeting with Abe. After the summit ends on Friday, Obama plans to visit the peace park in Hiroshima, becoming the first sitting U.S. president to visit the city on which the U.S. dropped an atomic bomb in 1945 in the closing days of World War II.

From left to right, U.S. President Barack Obama, Japan's Prime Minister Shinzo Abe, French President Francois Hollande, Britain's Prime Minister David Cameron, Prime Minister Justin Trudeau and European Commission President Jean-Claude Juncker take part in a planting ceremony on the grounds at Ise-Jingu Shrine in the city of Ise on the first day of the G7 leaders summit. (Stephane De Sakutin/AFP/Getty Images)